Silicon-on-insulator is the process of fabricating silicon based devices, such as complementary metal oxide semiconductor (CMOS) field effect transistors (FET) on top of a layer of electrically insulating material, such as an oxide. The layer of oxide is on top of a bulk silicon substrate in an integrated circuit (IC) chip and acts as an electrical barrier between the devices (e.g., FETs) and the bulk silicon. The layer of oxide greatly reduces electrical leakage from the devices, but also greatly reduces heat flow away from these devices. Accumulation of heat within a device, such as a FET, can reduce the performance and/or useful lifetime of the device.
Heat generation is a limiting factor to improving device operation in many technologies. For example, in radio frequency (RF) CMOS, switch performance is severely limited by the heat generated by the switch at high frequency. RF switches can use 200-300 mW of power, while an RF amplifier can use up to 3 W of power. This large amount of current flow over a relatively small region can result in heating of the devices up to 200° C. in the case of an RF amplifier. These elevated temperatures can significantly change the characteristics of the devices, as well as degrade the integrity of its construction materials. Since many of these devices are now built on SOI, the primary path for drawing heat away from the devices is through the electrical contacts formed over the devices. Further amplifying this problem is the desire to remove or reduce the number of electrical contacts in order to lower the capacitance of the devices, which will increase the need to dissipate heat because the electrical contacts do provide a path for the heat to escape. The RF parts affected by this issue are largely in cellular telephones.
Similar heat-related issues are present in the bipolar junction transistor (BJT) devices that are commonly used in radar and collision avoidance. SiGe-based BJT devices are driving toward increased operating frequencies of up to 300 GHz. As this frequency increases, the trapped residual heat plays a more prevalent role in degrading device performance. Passive structures, such as resistors, are also negatively affected by excess heat, which can affect the temperature coefficient of resistance.